In recent years, as to pneumatic radial tires for use in trucks and buses, it has been argued that dual tires be replaced with a single tire for the purpose of weight reduction and resource saving. The tread width of a single tire to be used instead of dual tires is increased while the outside diameter thereof is not changed, so that the aspect ratio of the tire is small. Accordingly, such a single tire is a so-called low-profile pneumatic radial tire. In addition, the inflation pressure is set higher than that of dual tires for obtaining the capability of the dual tires with the single tire.
In the case of a heavy-duty pneumatic radial tire having a wide tread width and a low aspect ratio as described above, the load on the belt layer in the tread portion at the time of running is significantly increased. This leads to a problem that the durability of the belt layer is reduced. For the purpose of reinforcing such a belt layer with a reduced durability, a number of measures as described below have conventionally been proposed. In such measures, a circumferential-direction reinforcement layer is formed by winding steel cords on a belt layer substantially in parallel to the tire circumferential direction. Then the circumferential-direction reinforcement layer is provided on a belt layer, so that the tread portion is provided with the rigidity in the circumferential direction (For example, Patent Documents 1 and 2).
However, steel cords have a high elastic modulus, and are thus vulnerable to compression. For this reason, when the steel cords are oriented in the tire circumferential direction as in the above-described circumferential-direction reinforcement layer, the steel cords are repetitively deformed when the tire is rolling so as to be likely to rupture due to fatigue. In particular, there has been a problem that fatigue rupture is likely to occur in steel cords in the edge portions of the circumferential-direction reinforcement layer to thus reduce the durability of the tire.
The inventor has made various studies on the reason why failure is likely to occur in the edge portions of the circumferential-direction reinforcement layer as described above. The inventor has thus found out the following reason. Specifically, the circumferential-direction reinforcement layer is subjected not only to a repetitive tensile strain when the tire is rolling. When in-plane bending of the belt layer occurs at the time of traveling corners, one of both edge portions is subjected to a tensile strain while the other one of both edge portions is subjected to a compressive strain. A series of such phenomena make failure likely to occur in the edge portions of the circumferential-direction reinforcing layer. On the basis of the finding, the inventor has eventually made the present invention.    Patent Document 1: Japanese Translation of PCT International Application No. 2001-522748    Patent Document 2: Japanese Translation of PCT International Application No. 2001-522748